This invention relates generally to an improved scrambling apparatus or encoder for self calibrating a selectively scrambled or encoded video signal. Currently, several techniques are known for scrambling a video signal. These techniques include, but are not limited to, selective inversion of at least a portion of the video signal, selective synchronization (sync) suppression and/or generation of split sync pulses. Typically, the sync pulse comprises a single pulse between the front and back porch characterized by a single voltage level for substantially the entire sync interval. A split sync pulse generally refers to a sync interval comprising two or more pulses, i.e., two or more voltage levels between the front and back porch. The desirability of providing split sync pulses and the advantages derived therefrom are fully described in the above referenced copending application. For purposes of illustration, an example of a video signal having a split sync pulse is shown in FIG. 1. It is to be understood that many other embodiments of a split sync signal will be readily apparent to one of ordinary skill in the art.
As shown in FIG. 1, the horizontal sync interval of the video signal comprises two signal portions. The first portion is preferably a pulse having a level at substantially -40 IRE corresponding to the sync tip level. The second portion of the sync interval preferably comprises a pulse having a level of substantially +100 IRE corresponding to the peak white level of the signal. Many other formats for split sync pulses are described in the referenced copending application and it is to be understood that split sync pulses may be used with selectively inverted video or non-inverted video signals and the split sync pulses may themselves be selectively inverted or non-inverted.
As more fully described in the referenced application, a signal transmitted with such a split sync signal has many advantages. For example, if it is desired to scramble the signal by inverting the video portion of the signal, the signal must be reinverted at the receiver and significantly, this inversion and reinversion must occur around the same axis of inversion in both the transmitter (scrambler) and receiver (descrambler). If it is desired to establish an axis of inversion substantially midway between the peak white level (e.g., +100 IRE) and the sync tip level (e.g., -40 IRE), it will be apparent that the axis of inversion should be +30 IRE. This axis of inversion can be calculated in the receiver by averaging the peak level and sync tip level. One way of selectively inverting signals or portions thereof is to produce substantially parallel signal paths in a scrambler comprising, e.g., a non-inverted signal path and an inverted signal path. Since there will be different components in the different signal paths which may introduce an offset of the signal in one path with respect to the signal in another path, it is important to ensure that the selected pulse levels of both the inverted and non-inverted signals produced in the scrambler are accurately calibrated, i.e., at least a portion of one of the signals substantially corresponds to at least a portion of another of the signals. Plural paths may be used in other scrambling applications as well. For example, they may be used to provide various levels of sync suppression.
In the past, scramblers were manufactured and subsequently calibrated in the factory using external calibration instruments known to those of ordinary skill in the art. This calibration was necessary to ensure that corresponding portions of the signals in each of the signal paths occurred at desired levels, e.g., in the case of split sync pulses it may be desired to have the low sync tip and high sync tip levels correspond to -40 IRE and +100 IRE levels respectively. This calibration step adds additional expense and time to the manufacture of scramblers which is obviously undesirable. To carry out this external calibration, the video signal in a non-inverted signal path might be sampled and compared to a video signal from an inverted path and any difference between a predetermined portion of the signals would be corrected.
As will be explained more fully below, the present invention relies on self-calibration. Self-calibration is distinguished from external calibration in that self-calibration is performed by circuitry within the scrambler itself and automatically adjusts at least a portion of one signal to correspond to a portion of another signal without the use of external calibration equipment.
As described in the cross-referenced application, video signals comprising a split sync pulse may be generated by using internally generated voltage levels that are multiplexed with a standard video signal to replace the normally occurring sync signal with an internally generated split sync signal. According to a preferred embodiment of the copending application, the split sync signal may comprise a pulse corresponding to the sync tip level (-40 IRE). It is also disclosed that it is desirable to perform the functions of D. C. clamping and automatic gain control (AGC). It is preferred tha the sync tip level be clamped to the -40 IRE voltage level and then the gain be adjusted so that the back porch level corresponds to the O IRE level. If clamping and gain control do not occur prior to insertion of internally generated split sync pulses, errors can occur if, e.g. the clamped sync tip level is not substantially the same as the internally generated low sync tip level portion of the sync pulse.